1. Field of the Invention
This invention relates to electronic circuits used to current-limit the outputs of power supplies and more specifically to circuits used to limit the output current of voltage regulators or other similar circuits.
2. Description of the Relevant Art
Voltage regulators are designed to provide a constant voltage over a variety of load impedances. As the impedance of the load increases, the voltage regulator requires less output current to keep the load at a constant voltage. Conversely, as the impedance of the load decreases, more current is required to maintain the same constant voltage. The problem addressed by this invention is encountered in voltage regulator circuits when the output current required to maintain a constant voltage is greater than the safe operating condition of the pass (output) transistors of the voltage regulator. Therefore, it is common for voltage regulator circuits to have over-current protection to limit the output current to a safe operating condition.
FIG. 1, shows the output of a voltage regulator with a over-current protection as is known in the prior art. The circuit operates by error amplifier 10 receiving a reference voltage, V.sub.trk. The reference voltage V.sub.trk is the desired output voltage of the voltage regulator circuit 8. Error amplifier 10 drives the base of the pass transistor 14 proportional to the amount of current necessary to maintain the output, V.sub.out, of the voltage regulator at the V.sub.trk voltage. If V.sub.out begins to fall below V.sub.trk, the output of the error amplifier 10 rises which increases the base voltage of pass transistor 14 thereby driving more current into the V.sub.out node which raises the V.sub.out voltage.
The over-current protection circuit consists of current source 12 and transistor 16, and sense resistor 18. Sense resistor 18 is typically a very low resistance resistor which can handle the large currents of the pass transistor 14. As the current through transistor 14 and resistor 18 increases, the voltage drop across sense resistor 18 increase. Therefore, the resistance of sense resistor can be selected so that transistor 16 turns on when the current through sense resistor 18 reaches an unsafe operating current for any component of the voltage regulator circuit 8. As the load current increases, the voltage drop across resister 18 causes transistor 16 to begin to conduct. The collector current of transistor 16 shunts away available base current for transistor 14 supplied by current source 12 thereby limiting the output current (the output current is the base current.times.the beta of the transistor, as is known in the art). As output load increases, the base current for transistor 14 decreases. The characteristics of current source 12, pass transistor 14, and transistor 16 can be selected to limit the maximum current transistor 14 can deliver to a load. Thus, transistor 16 and resistor 18 limit the output current in transistor 14 during an over-current condition by controlling the base current to transistor 14.
As an example to illustrate the operation of the prior art circuit in FIG. 1, the safe operating current of pass transistor 14 may be limited to 1 amp and transistor 16 may be forward biased at around 0.7 volts. Then, a sense resistor of around: EQU 0.7 volts/1.amp=0.7 ohms
would be required for the over-current protection circuit to limit the current to 1 amp. At about one amp, the voltage across sense resistor 18 is around 0.7 volts. Thus, transistor 16 begins to shunt the current from the base of pass transistor 14 which consequently limits the current through the pass transistor 14 to the save operating current.
In the prior art circuit of FIG. 1, the sense resistor 18 is required to detect the over-current condition. As current flows through the sense resistor 18, the resulting voltage drop can be problematic since power is dissipated in the chip, since load regulation is deteriorated, and drop-out voltage is increased. Additionally, a sense resistor is undesirable since it requires a significant amount of area on an integrated circuit.
FIG. 2, shows a second voltage regulator with an over-current protection as is also known in the prior art. Like FIG. 1, voltage regulator 40 has an error amplifier 10 for receiving a V.sub.trk voltage and a pass transistor 14. However, voltage regulator 40 does not have a sense resistor 18.
Voltage regulator 40 operates by error amplifier 10 driving pass transistor 14 in response to the difference in voltage between V.sub.trk and V.sub.out. The lower the voltage V.sub.out is relative to V.sub.trk, the higher the voltage on the gate, relative to the source, of pass transistor 14 and thus the more current driven through pass transistor 14.
In voltage regulator 40, the over-current protection circuit includes transistors 22, 24, 26, 28, 34, and 36, current source 30, and capacitor 32. The gate of transistor 24 is connected to the output of error amplifier 10 and to the gate of pass transistor 14. Consequently, a current flows through transistor 24 which is proportional to the current through transistor 14. The proportion is determined by the ratio of the relative sizes of the two transistors, as is well known in the art. The current through transistor 24 is mirrored by transistor 36 to 34. Current source 30 provides a reference current which is mirrored by transistors 26 and 28 and, thus, transistor 28 acts as an active load to transistor 34. Capacitor 32 acts as the compensation capacitor and may be necessary to avoid oscillations on this node. Transistor 22 is controlled by the voltage drop across transistor 28 which is controlled by the current through transistor 34 since the gate of transistor 22 is connected to drain of transistors 28 and 34.
In operation, error amplifier 10 regulates the output voltage V.sub.out by controlling the current through transistor 14 by controlling the voltage on the gate of transistor 14. The current through transistor 14 is scaled down and transmitted through transistor 24 since the gate of transistor 24 an 14 are connected together. The current through transistor 24 is mirrored by transistor 36 and 34. At the same time, current source 30 provides a reference current which is mirrored by transistors 26 and 28. Therefore, transistor 28 acts like a load resistor to the drain of transistor 34. When the output current is low, the current in transistors 24, 36, and 34 is relatively low and thus the voltage drop across transistor 28 is not large enough to turn on transistor 22. Hence, transistor 14 is controlled by error amplifier 10. Conversely, when the output current is high, the currents in transistors 24, 34, and 36 is high which creates a large voltage drop across transistor 28. Thus, transistor 22 is driving the gate of transistor 14 to a high voltage thereby limiting the current flow through transistor 22.
It has been observed that this circuit requires additional circuitry over circuit 8 and requires capacitor 32 to ensure stability (no oscillations) during current limiting.